The only case of double headers I can think of is when using scripts
that set a content-type, then try to set it again and the language
itself don't prevent it.
I think the "right" option in such case would be to follow the last one,
as it's the one provided nearer the content.
So I vote for using the last one.
PS: Interestingly, this behavior could be used to send xhtml (and get it
shown on IE) without sending different headers to each browser:
Content-Type: text/html
Content-Type: application/xhtml+xml